my $t = 1;
grep { $t^=1 } LIST

##</code><code>##

my $t = 1;
grep { $t = 1-$t } LIST

##</code><code>##

my $t = 1;
grep { ++$t % 2 } LIST

##</code><code>##

map { $_%2 ? $ARRAY[$_] : () } 0..$#ARRAY

##</code><code>##

>perl -le"$t=1; print grep { $t^=1 } qw( a b c d e );"
bd

##</code><code>##

>perl -le"$t=1; print grep { $t = 1-$t } qw( a b c d e );"
bd

##</code><code>##

>perl -le"$t=1; print grep { ++$t % 2 } qw( a b c d e );"
bd

##</code><code>##

>perl -le"@a = qw( a b c d e ); print map { $_%2 ? $a[$_] : () } 0..$#a;"
bd

##</code><code>##

my $t = 0;
grep { $t^=1 } LIST

##</code><code>##

my $t = 0;
grep { $t=1-$t } LIST

##</code><code>##

my $t = 0;
grep { ++$t % 2 } LIST

##</code><code>##

map { $_%2 ? () : $ARRAY[$_] } 0..$#ARRAY

##</code><code>##

>perl -le"$t=0; print grep { $t^=1 } qw( a b c d e );"
ace

##</code><code>##

>perl -le"$t=1; print grep { $t = 1-$t } qw( a b c d e );"
ace

##</code><code>##

>perl -le"$t=0; print grep { ++$t % 2 } qw( a b c d e );"
bd

##</code><code>##

>perl -le"@a = qw( a b c d e ); print map { $_%2 ? () : $a[$_] } 0..$#a;"
ace

##</code><code>##

my $t = 1;
map { ($t^=1) ? EXPR_FOR_ODD : EXPR_FOR_EVEN } LIST

##</code><code>##

my $t = 1;
map { ($t = 1-$t) ? EXPR_FOR_ODD : EXPR_FOR_EVEN } LIST

##</code><code>##

my $t = 1;
map { (++$t % 2) ? EXPR_FOR_ODD : EXPR_FOR_EVEN } LIST

##</code><code>##

map { $_%2 ? EXPR_FOR_ODD : EXPR_FOR_EVEN } 0..$#ARRAY

##</code><code>##

>perl -le"$t=1; print map { ($t^=1) ? uc : lc } qw( a b c d e );"
aBcDe

##</code><code>##

>perl -le"$t=1; print map { ($t=1-$t) ? uc : lc } qw( a b c d e );"
aBcDe

##</code><code>##

>perl -le"$t=1; print map { (++$t % 2) ? uc : lc } qw( a b c d e );"
aBcDe

##</code><code>##

>perl -le"@a = qw( a b c d e ); print map { $_%2 ? uc($a[$_]) : lc($a[$_]) } 0..$#a;"
aBcDe